Support busybox 'date' command

BB's `date` doesn't support the long form of this arg (`--utc`), only
the short form, while coreutils supports both versions. By using the
short form swclock-offset can support both versions of `date`

src/swclock-offset-boot.sh

@@ -33,4 +33,4 @@ offset_epoch=$(cat $offset_file)
 swclock_epoch=$((hwclock_epoch + offset_epoch))
 
 # set system time, dump output
-date --utc --set=@$swclock_epoch > /dev/null
+date -u --set=@$swclock_epoch > /dev/null

src/swclock-offset-shutdown.sh

@@ -26,7 +26,7 @@ if [ ! -d $offset_directory ]; then
 fi
 
 # calculate offset
-swclock_epoch=$(date --utc +%s)
+swclock_epoch=$(date -u +%s)
 hwclock_epoch=$(cat $rtc_sys_node)
 offset_epoch=$((swclock_epoch - hwclock_epoch))